The present invention relates to a sheet feeding device that separates a sheet from a plurality of stacked sheets one sheet by one sheet and conveys to an image forming section or to an image reading section, in an image forming apparatus or an image reading apparatus.
As the sheet feeding device of this kind, those each being composed of a pickup roller, a feed roller and a retarding roller as is disclosed in, for example, Unexamined Japanese Patent Application Publication No. 5-43073, are widely used.
In this sheet feeding device, a sheet located at the outermost position of stacked sheets is caused to receive conveyance force by a pickup roller to be fed out and is conveyed after being separated from others by a feed roller and a retarding roller.
In the Unexamined Japanese Patent Application Publication No. 5-43073, between the pickup roller and the feed roller, there are provided guide members wherein a distance between the guide members arranged to interpose a sheet conveyance path between them becomes narrower gradually toward the downstream side from the upstream side in the sheet conveyance direction.
The sheet feeding device described in the Unexamined Japanese Patent Application Publication No. 5-43073 has a problem that conveyance of a thick sheet is not smooth, and sheet feeding failure tends to be caused.
Problems generated in the case of a thick sheet will be described as follows, referring to FIGS. 1 (a) and 1 (b).
When sheet S is left under low humidity environment for a long time, moisture evaporates from the sheet, and an amount of evaporation on a peripheral portion of the sheet is more than that on the central portion, resulting in occurrence of a curvature whose form is close to that of a part of a spherical surface on a surface of sheet S. As a result, when a sheet S is placed on a sheet feeding table, a lift that is shown with K is formed on each of both end portions of the sheet S.
FIG. 1 (b) shows a phenomenon that takes place in the sheet feeding for curved sheet S shown in FIG. 1 (a).
As is shown in FIG. 1 (b), leading edges K on both lifted end portions of the sheet S hit an inclined guide surface of the guide member 110, when Y represents the direction of conveyance for a sheet in sheet feeding. In the case of a thick sheet, the sheet S that hits the guide surface does not bend along the guide surface because stiffness of the sheet S is high, and thus, a leading edge of the sheet S is blocked by the guide member 110, which stops conveyance of the sheet S.
In the case of a thin sheet whose stiffness is low, a stop of conveyance of this kind hardly takes place, but when stiffness is high as in the case of a thick sheet, a stop of conveyance that is called no-feed tends to take place.
An objective of the invention is to solve the aforesaid problem in a conventional sheet feeding device, and thereby to provide a sheet feeding device that can feed sheets stably even in the case of a thick sheet.